1. Technical Field
The present invention generally relates to electrical circuits and in particular to power supply systems in electrical circuits.
2. Description of the Related Art
In most microprocessor chips and other on-chip electrical devices, on-chip switching activities of transistors and other components generate switching current(s), I(t). These switching currents cause noise in the power supply system. The power supply systems generally consists of: (1) an on-chip decoupling capacitor (decap) “C_onchip”; and (2) a package/board with other decap components and voltage regulator module(s) (VRM). Of these two mechanisms that make up the power supply system, C_onchip is the most efficient decoupling contributor for reducing noise because of the proximity of C_onchip to the switching transistors. However, the amount (i.e., the capacitance) of C_onchip is limited. For example, in one typical microprocessor chip, the capacitance associated with C_onchip is ˜0.2 uF, while, for comparison, package decaps have capacitance of 66 uF.
When a large “dI/dt” (i.e., rate of change of current) event occurs and charges are needed by transistors, C_onchip acts as a local battery to supply the charges. On-chip capacitance stores total charge “Qtotal” (where Qtotal=Vo*C_onchip, where Vo is the nominal voltage and “*” refers to the multiplication operation). But only a charge of “Q” may be supplied to the transistors. Here, Q=ΔV*C_onchip, where the symbol (delta) “A” represents “a change in”. Thus, ΔV refers to a change in the value of V, or voltage “droop”). Usually, ΔV/Vo is required to be less than 10%; otherwise, the chip may not function properly and operation at a maximum operating frequency “fmax” may be compromised. This means that, at most, only 10% of the charges stored on C_onchip is utilized in mitigating noise.
On-chip capacitance comes from structures that occupy precious on-chip estate. In the BE chip, C_onchip comes from gate capacitance of transistors dedicated to decoupling. To increase the amount of charge that C_onchip is able to supply, one obvious but costly way is to increase the size of C_onchip. However, a larger C_onchip means increasing the size of the integrated circuit(s).